Describe the bug I found docstring of format_str() is confusing

As is

    `mode` determines formatting options, such as how many characters per line are
    allowed.  Example:

    >>> import black
    >>> print(black.format_str("def f(arg:str='')->None:...", mode=Mode()))
    def f(arg: str = "") -> None:
        ...

Issue the above code causes NameError

#4478 included some initial support for the new ConstBlock expression kind variant, but there's opportunities to enhance and improve the formatting.

The relevant section of the codebase can be found in formatting/expr.rs
https://github.com/rust-lang/rustfmt/blob/cbe01e47024c852e2ed852671e76f7929c4bab91/src/formatting/expr.rs#L127-L129

There are 3 core activities to completing this issue:

When I try to beautify the i get the following problem
TypeError: Cannot read property 'on' of undefined

I just installed beautify and uncrustify

https://gist.github.com/ODCA117/917affd6b11437517535a45350379820

We need to update our README.md file before the 1.0 release.